This invention relates generally to optical amplifiers and more particularly, to an in-line optical amplifier that can be coupled to optical fibre, wherein the amplifying medium has a substantially larger mode field diameter than the optical fibre to which it is coupled.
There is considerable interest in using rare earth doped fiber amplifiers to amplify weak optical signals for both local and trunk optical telecommunications networks. The rare earth doped optical amplifying fibers exhibit low-noise, have relatively large bandwidth with low polarization dependence, substantially reduced crosstalk problems, and low insertion losses at the relevant operating wavelengths which are used in optical communications. Furthermore, rare earth doped optical fiber amplifiers can be coupled end-to-end to a transmission fiber, and coupled, through a directional coupler, to a laser diode pump. The directional coupler is designed to have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength so that maximum pump energy is coupled to the amplifier with minimal signal loss. When the amplifying medium is excited with the pump laser, signal light traversing the amplifier experiences gain. The pump energy may be made to propagate either co-directionally or counter-directionally relative to the signal energy, selected for higher power efficiency or better noise performance
To date, erbium fiber amplifiers appear to have the greatest potential for the high amplification necessary to overcome the signal losses. Erbium doped fiber amplifiers (EDFAs) operate at 1550 nm which is of particular interest for optical communication systems because, in this wavelength region, the amplifiers exhibit low insertion loss, broad gain bandwidth (approximately 30 nm) and relatively polarization insensitive gain.
Such amplifiers, pumped with light having a wavelength of 980 nm can have a gain as high as 26 dB but require as much as 76 mW of launched pump power. It has generally been desired to achieve a higher gain together with a lower value of pump power coupled into a fiber, and such optimization of EDFAs has been a goal. The pump required to launch a signal into a single mode fibre is quite costly.
The present invention has realized a design to utilize a very high power pump launching a multimoded signal having approximately 1 W of pump power. Currently, high power optical pump lasers are commercially available at a relatively low cost. Such high power pumps are not compatible for use with erbium doped fibre in the manufacture of EDFAs. However, this invention provides a relatively inexpensive optical amplifier that is compatible for use in an optical fibre telecommunications system or for other uses.
This invention also provides a device that does not require unwieldy lengths of erbium doped fibre to form an amplifier. In contrast, the instant invention uses a block of glass having a mode field diameter orders of magnitude larger than the mode field diameter of erbium doped fibre.
By enlarging the mode field of the signal beam, greater pump energy can be applied without the significant difficulty and loss which are present when coupling pump energy into a single mode fiber amplifier.
By using a block of glass having a rare earth therein, packaging, temperature stabilizing and temperature tuning of the amplifier become practicable.
Furthermore, a cylindrical block of glass having planar ends, lends itself to applying coatings or filters thereto, thereby forming selective filters at ends of the erbium doped block to allow the pump light in, and the signal light in at opposite ends, while preventing light at the pump wavelength to propagate out with the amplified signal.
In accordance with the invention there is provided, an optical amplifier comprising: an optical waveguide for carrying an optical signal to be amplified, the optical waveguide having an output end for outcoupling the optical signal;
a substantially collimating lens optically coupled with the output end of the optical waveguide for receiving the optical signal and for providing a substantially collimated beam to be amplified, the substantially collimated beam having a substantially larger mode field diameter than the optical signal being carried by the optical waveguide;
a block of light transmissive material sized to carry the substantially collimated beam for amplification, the block of light transmissive material being comprised of a gain medium doped with a rare-earth element, the block being disposed to receive the substantially collimated optical beam; and,
a high power pump disposed to impart optical energy to the block; and, an output optical waveguide disposed to couple focused light of the optical signal after it has been amplified within the block of light transmissive material.
In accordance with the invention there is further provided, an optical amplifier comprising: a first optical waveguide for providing a signal to be amplified, the waveguide having an average mode field diameter d1;
a second optical waveguide optically coupled with the first waveguide for receiving the signal after it has been amplified, the second waveguide having a mode field diameter d2, where d1, and d2 are substantially smaller than d3;
a light transmissive amplifying medium for guiding a beam having a mode field diameter of at least d3, said light transmissive amplifying medium being disposed to receive light from the first optical waveguide and to provide amplified light to the second optical waveguide;
a pump optically coupled with the light transmissive amplifying medium for providing pump energy to the amplifying medium.
In accordance with the invention there is further provided, an optical amplifier for amplifying an incoming optical signal comprising a glass block in the form of a light transmissive medium sized to carry a beam having a mode field diameter of at least 100 xcexcm, the block being doped with a rare earth for amplifying light passing therethough when the rare earth is excited by a pump beam, the light transmissive medium having a filter at an end thereof for passing the pump beam and for substantially preventing the optical signal to be amplified from passing therethrough, and having a filter at another end thereof, for passing the signal to be amplified and for substantially preventing the pump beam from passing therethrough.
In accordance with the invention there is provided, an optical amplifier comprising:
an input optical fibre for providing a signal to be amplified;
an amplifying medium comprising a light transmissive material having a diameter substantially greater than the diameter of the input optical fibre, for receiving the signal to be amplified;
a lens for substantially expanding a mode field diameter of a beam of light of the signal to be amplified, optically coupled between the input waveguide and the amplifying medium;
a pump source for providing high intensity optical pump energy to the amplifying medium; and,
an output optical fibre for receiving an amplified optical signal from the amplifying medium.
In accordance with another aspect of the invention a method of amplifying an optical signal is provided, comprising the steps of:
coupling the optical signal from an optical fibre into an amplifying medium having a diameter a plurality of orders of magnitude greater than a mode field diameter of the signal propagating with the optical fibre such that the mode field diameter of the signal is converted to a substantially larger collimated beam than the signal propagating with the optical fibre;
pumping optical energy having a different wavelength from the optical signal into the amplifying medium, and receiving the amplified optical signal from the amplifying medium.
In accordance with the invention there is further provided, a method of amplifying an optical signal comprising the steps of:
launching a beam carrying the optical signal from an optical fibre;
substantially increasing a mode field diameter of the beam and providing the beam to an amplifying medium;
pumping optical energy having a different wavelength from the optical signal into the amplifying medium, and receiving an amplified optical signal from the amplifying medium and,
decreasing the mode field diameter of the amplified signal and coupling the amplified signal to an output optical fibre.